dvbscan

scan is one of the command line utilities contained in the LinuxTV dvb-apps. When using scan, one indirectly instructs it to search for programs streams that can be received from specified frequencies and generates a resultant channel list.

Note: Depending on where/how you got the dvb-apps package, the program may be called either scan, scandvb or dvbscan. In the following examples, we'll use the name dvbscan.

Most experts or experienced users will likely find that this information conveys sufficient details in so far as how to manipulate a specific DVB hardware device with dvbscan. 'New users', however, are probably left scratching their heads by some of the options listed in the output -- but fear not!

One thing that is noteworthy from the output above is that dvbscan does not take file system device names; instead, it cobbles them together from the adaptor, frontend and demux numbers. If you only have one tuner, you don't need to worry about this detail.

Scanning for channels you can receive

Running dvbscan with an initial scan file

One of the ways that dvbscan can be used to search for channels that you can receive is by instructing it to use an initial scan file. An initial scan file provides a list of frequencies, and some other specific data that dvbscan should use, to proceed with a scan.

The LinuxTV dvb-apps package supplies a large number of user submitted initial scan files for many different localities. The exact directory location where these files are installed may vary depending upon the installation of the package (for an explanation of this incongruity, see here). Nonetheless, the file you're looking for is:

First, contained within the folder for the type of DVB device you're using. For example, DVB-T users should find an initial scan file from /usr/share/dvb/dvb-t/ ... other installations may use:

/usr/share/doc/dvb-utils/examples/scan/dvb-t/ or

/usr/share/doc/dvb-apps/dvb-t or

/usr/local/share/dvb/scan/dvb-t/ or ...

Second, the name of the file you're looking for is of the form cc-Ttttt, where cc is a two-letter country abbreviation, and Ttttt is the name of the location of the transmitter. So, if you're in:

Adelaide, Australia, you'd want to look for a file called au-Adelaide (eg. /usr/share/dvb/dvb-t/au-Adelaide)

Oxford, UK you would use uk-Oxford (eg. /usr/share/dvb/dvb-t/uk-Oxford).

Obviously, you will have to adapt this command for your own situation/locality. In any regard, dvbscan will try to find programs on frequencies it takes from the initial scan file (e.g. au-Adelaide or uk-Oxford, or where ever ...).

If you can't find a suitable initial scan file to use with dvbscan and:

If you're an North American user:

Just use the initial scan file entitled "us-ATSC-center-frequencies-8VSB" (for Over-the-Air) or "us-Cable-Standard-center-frequencies-QAM256" (for digital cable), as these will lead dvbscan to proceed to do a full search of the channel frequency spectrum.

Create your own initial scan file

During the scan

After initiating the dvbscan command, a lot of output data will be produced in the shell. The first thing that dvbscan does is to report which file it is parsing information from and with what hardware device that this information is going to be utilized.
For example:

Next dvbscan commences a search through all the frequencies listed in an initial scan file. During this phase, the utility will update the progress of the scan within its output. Do note that this process can take some time to complete, particularly if the initial scan file contains a long list of frequencies to test. The information presented provides details on the channels being detected during the scan. Often many lines of transponder information are reported. Here are some examples:

This is the tuning information for the first transponder. Each transponder can transmit multiple programs and further discussion about their contents is provided later below. Note that the information presented doesn't really say if you can actually make use of the streams that are detected. This is particularly true in the case of cable systems, which tend to encrypt most channels. So while dvbscan is perfectly capable of detecting such channels, you may very well be out of luck on the viewing end.

Do not be alarmed by such "tuning failed" messages, as this is not fatal. It simply means that no signal was found on that particular frequency -- it's possible that the initial scan files contain information about stations that are not yet, or no longer, in service, or it may imply that you presently lack a strong enough signal for reception to occur.

After the scan completes

Eventually, upon completion of its search, dvbscan outputs a list of services that it found.

Note: The actual number of available services it finds will, of course, depend upon several factors (the number of services available in your area and those related to signal strength). Check dvbscan's output with a list of known services available for your area. If you are missing some services, then try to troubleshoot your antenna and/or cable configuration. Hopefully you will be able to improve the environment for signal reception. If you are unable to obtain a particular service, then you may just be out of range or subject to inhospitable environmental conditions that your hardware just cannot overcome to achieve reception. If zero services where found, perhaps check your antenna and/or cable lead ... maybe it's simply a case that its not plugged in at all ;-)

Each line in the output consists of a number of fields delimited by colons. The exact number of fields will depend upon the type of scan you where performing, as will be exemplified below. In any regard, these lines of output are szap/tzap/czap/azap compatible and, as such, can be copied and pasted into a simple text editor to create a "colon-separated" channels.conf file. Alternatively, one can automate the creation of a channels.conf file as described later below.

The frequencies shown here in Hz correspond to 226.5 MHz for ABC and 219.5 MHz for TEN. Note that the first two channels are the same frequency, which means that they're both present in the same MPEG transport stream, but are distinguished by their respective PIDs. Looking at the values, it's clear that ABC DiG Jazz is a radio station, since it doesn't have a video PID. Less clear is why the ABC HDTV stream (a known TV channel) does not have an audio PID. This is a real issue: this means that when tuning to that stream, no audio is output. We'll look at how to solve this problem later below.

Notes:

These fields will vary depending on the type of modulation; the ones shown are for QFDM (and, paradoxically, not for QAM). For other forms of modulation, the meanings of the fields differ. See the source code (and document it, please!) at util/scan/dump-zap.c.

If a channel has a video PID of 0 and the audio PID is not 0, then it's probably a radio channel.

if dvbscan detected a channel but the signal was too weak for reliable reception, the video and audio PIDs (third- and second-last numbers in the output line) will likely be shown as 0.

So, after reviewing the above information, you should now be able to determine the pertainent information from the output of any scan. As a further example,

conveys that the channel BBC ONE is found on frequency 578000000 Hz, and has the two PIDs 600 and 601 for the video and audio stream.

Sample output from an ATSC scan:
There aren't as many colon delimited fields in the output as there would be as in the case of a scan of DVB-{C,T,S} system counterparts, but the substance of the output is largely the same, as can be seen in the following sample list:

Creating a channels.conf file

Next create a "channels.conf" file. This file takes the data for your available TV-channels, as shown above, and stores it for tuning use by simple "zapper" programs, such as tzap, or for higher level applications like MPlayer or Xine.

Each of the "zapper" utilities will, by default, expect to find the channels.conf file in the specific hidden (dotted) directory off your "home" directory. For example, for DVB-C tuning with czap, you would set up a ~/.czap directory. Likewise, for use with szap, you would make a ~/.szap directory, and so forth. Here are some examples of how to automagically create the channels.conf file by directing the stdout from the dvbscan command to be placed in a file in a given directory:

one step scan and channels.conf creation (i.e if you were paying attention, don't need to run dvbscan twice as was like performed above)

~/.{a,c,s,t}zap directories for the channels.conf is just the default location. Each of the zapper utilities can use any other location for channels.conf provided you specify its location ... (i.e. ~/. is really just a ease of use) ... see the zap articles for more usage detail

the dvb-apps package contains a number of files that are essentially channels.conf files for given localities. These will probably be installed on you system. For example, /usr/local/share/dvb/zap/dvb-t/au-Adelaide is a canned version of the output of dvbscan. It may be out of date, so it's better to generate it from dvbscan, but if you can't get dvbscan to work, you might get further by using this file.

An alternative method of using dvbscan to check a specific frequency

Tune to that specific frequency (e.g. using szap/tzap/czap/azap) and then use './dvbscan -c' or './atscscan -c'.

A word about dvbscan and audio streams

scan does not always seem to correctly recognize audio streams for a television channel.

Example, in Australia, most channels broadcast HDTV with AC3 audio, which scan does not recognize when invoked as above. The result is the output shown above for ABC HDTV: no audio.

This is almost certainly a bug in scan. Until it is fixed, you can proceed in the following way:

This output was limited to the interesting information. The last two lines show the output for ABC HDTV and ABC TV Adelaide. ABC TV Adelaide has a video pid 0x200, an audio pid 0x28a, and an AC3 pid 0x294. The meanings of the other values still need to be documented.

By contrast, ABC HDTV has a video pid 0x90a, and AC3 pid 0x90b--and no audio pid. This is the problem. In the output from scan based on the transmitter data, the corresponding values are (leaving out the middle of the lines, which aren't of interest here):

ABC HDTV: ... :2314:0:592
ABC TV Adelaide: ... :512:650:593

As described above, the third-last value is the video pid, and the second-last value is the audio pid. In this output they're in decimal, while in the output of dvbscan -c they're in hexadecimal; but they correspond.

To work around the lack of audio pid in ABC HDTV, convert the value of the AC3 pid (0x90b) to decimal (2315) and edit the channels.conf file: